Line comments after null value used as argument in function call are removed

[rehael]

Issue created from fantomas-online

Code

let v = f null // comment

let lostComment =
    f // comment
        null // comment
        v // comment
        null // comment
        42 // comment
        null // comment
    // comment

Result

let v = f null

let lostComment =
    f // comment
        null
        v // comment
        null
        42 // comment
        null
// comment

Problem description

Comments after null are removed – should be left as is, as it is for other argument types. Looks like null is handled specially.

I was trying to find how it is handled in Fantomas' code, but clearly my fsharp-fu is not enough to grasp it, so probably can't help directly with making a PR/fix. :(

Extra information

• The formatted result breaks by code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas Master at 04/17/2021 11:23:36 - 26674f9

Default Fantomas configuration

_{Did you know that you can ignore files when formatting from fantomas-tool or the FAKE targets by using a .fantomasignore file?}

[nojaf]

Hello, thank you for reporting this issue.
Comments are not part of the F# Abstract Syntax tree.
To overcome this limitation, Fantomas uses a concept called "trivia".
In short, trivia is the detection of missing information in the F# tokens.
After finding those trivia, we try to assign them to existing nodes of the tree.
See https://www.youtube.com/watch?v=FCrkUqCCzgI&ab_channel=nojaf for more information.

In this specific scenario, the comments are found and assigned to a SynExpr_Null node.
You can see this in the "trivia" tab of the online tool.

The problem you are facing is that the trivia is not printed in the CodePrinter.

fantomas/src/Fantomas/CodePrinter.fs

Lines 2657 to 2694 in 728e31e

	|> (match synExpr with
	| SynExpr.App _ -> genTriviaFor SynExpr_App synExpr.Range
	| SynExpr.AnonRecd _ -> genTriviaFor SynExpr_AnonRecd synExpr.Range
	| SynExpr.Record _ -> genTriviaFor SynExpr_Record synExpr.Range
	| SynExpr.Ident _ -> genTriviaFor SynExpr_Ident synExpr.Range
	| SynExpr.IfThenElse _ -> genTriviaFor SynExpr_IfThenElse synExpr.Range
	| SynExpr.Lambda _ -> genTriviaFor SynExpr_Lambda synExpr.Range
	| SynExpr.ForEach _ -> genTriviaFor SynExpr_ForEach synExpr.Range
	| SynExpr.For _ -> genTriviaFor SynExpr_For synExpr.Range
	| SynExpr.Match _ -> genTriviaFor SynExpr_Match synExpr.Range
	| SynExpr.MatchBang _ -> genTriviaFor SynExpr_MatchBang synExpr.Range
	| SynExpr.YieldOrReturn _ -> genTriviaFor SynExpr_YieldOrReturn synExpr.Range
	| SynExpr.YieldOrReturnFrom _ -> genTriviaFor SynExpr_YieldOrReturnFrom synExpr.Range
	| SynExpr.TryFinally _ -> genTriviaFor SynExpr_TryFinally synExpr.Range
	| SynExpr.LongIdentSet _ -> genTriviaFor SynExpr_LongIdentSet synExpr.Range
	| SynExpr.ArrayOrList _ -> genTriviaFor SynExpr_ArrayOrList synExpr.Range
	| SynExpr.ArrayOrListOfSeqExpr _ -> genTriviaFor SynExpr_ArrayOrListOfSeqExpr synExpr.Range
	| SynExpr.Paren _ -> genTriviaFor SynExpr_Paren synExpr.Range
	| SynExpr.InterpolatedString _ -> genTriviaFor SynExpr_InterpolatedString synExpr.Range
	| SynExpr.Tuple _ -> genTriviaFor SynExpr_Tuple synExpr.Range
	| SynExpr.DoBang _ -> genTriviaFor SynExpr_DoBang synExpr.Range
	| SynExpr.TryWith _ -> genTriviaFor SynExpr_TryWith synExpr.Range
	| SynExpr.New _ -> genTriviaFor SynExpr_New synExpr.Range
	| SynExpr.Assert _ -> genTriviaFor SynExpr_Assert synExpr.Range
	| SynExpr.While _ -> genTriviaFor SynExpr_While synExpr.Range
	| SynExpr.MatchLambda _ -> genTriviaFor SynExpr_MatchLambda synExpr.Range
	| SynExpr.LongIdent _ -> genTriviaFor SynExpr_LongIdent synExpr.Range
	| SynExpr.DotGet _ -> genTriviaFor SynExpr_DotGet synExpr.Range
	| SynExpr.Upcast _ -> genTriviaFor SynExpr_Upcast synExpr.Range
	| SynExpr.Downcast _ -> genTriviaFor SynExpr_Downcast synExpr.Range
	| SynExpr.DotIndexedGet _ -> genTriviaFor SynExpr_DotIndexedGet synExpr.Range
	| SynExpr.DotIndexedSet _ -> genTriviaFor SynExpr_DotIndexedSet synExpr.Range
	| SynExpr.ObjExpr _ -> genTriviaFor SynExpr_ObjExpr synExpr.Range
	| SynExpr.JoinIn _ -> genTriviaFor SynExpr_JoinIn synExpr.Range
	| SynExpr.Do _ -> genTriviaFor SynExpr_Do synExpr.Range
	| SynExpr.TypeApp _ -> genTriviaFor SynExpr_TypeApp synExpr.Range
	| SynExpr.Lazy _ -> genTriviaFor SynExpr_Lazy synExpr.Range
	| _ -> id)

SynExpr_Null should be added there.

Are you interested in submitting a PR for this?
If so, please read our Contribution Guidelines carefully.

[rehael]

I will try understanding this and working on PR – in case of any problems I will ask here.

BTW: Similar issue with comment removal in pattern match:

Before:

match v with
| x
| y // comment
| z -> 42
| _ -> 0

After:

match v with
| x
| y
| z -> 42
| _ -> 0

[nojaf]

That would be the same category of problems yet there would not be one fix that would solve both problems.
So, I encourage you to open a new issue for each case you encounter (as mentioned here).